Abstract
Ten organoimido polyoxometalate (POM)-based chromophores have been synthesized and studied by hyper-Rayleigh scattering (HRS), Stark and Resonance Raman spectroscopies, and density functional theory (DFT) calculations. HRS β0 values for chromophores with resonance electron donors are significant (up to 139 × 10-30 esu, ∼5 times greater than that of the DAS+ cation), but systems with no donor, or the -NO2 acceptor show no activity, in some cases, despite large DFT-predicted β-values. In active systems with short (phenyl) π-bridges, β0 values comfortably exceed that of the purely organic structural analogue N,N-dimethyl-4-nitroaniline (DMPNA), and intrinsic β-values, β0/N3/2 (where N is the number of bridge π-electrons) thus appear to break empirical performance limits (β0/N3/2 vs λmax) for planar organic systems. However, β0 values obtained for extended systems with a diphenylacetylene bridge are comparable to or lower than that of their nitro analogue, N,N-dimethyl-4-[(4-nitrophenyl)ethynyl]-aniline (DMNPEA). Resonance Raman spectroscopy confirms the involvement of the POM in the electronic transitions, whether donor groups are present or not, but Stark spectroscopy indicates that, in their absence, the transitions have little dipolar character (hence, NLO inactive), consistent with DFT-calculated frontier orbitals, which extend over both POM and organic group. Stark and DFT also suggest that β is enhanced in the short compounds because the extension of charge transfer (CT) onto the POM increases changes in the excited-state dipole moment. With extended π-systems, this effect does not increase CT distances, relative to a -NO2 acceptor, so β0 values do not exceed that of DMNPEA. Overall, our results show that (i) the organoimido-POM unit is an efficient acceptor for second-order NLO, but an ineffective donor; (ii) the nature of electronic transitions in arylimido-POMs is strongly influenced by the substituents of the aryl group; and (iii) organoimido-POMs outperform organic acceptors with short π-bridges, but lose their advantage with extended π-conjugation.
Highlights
Polyoxometalates (POMs) are a class of anionic, molecular metal oxide clusters, whose range of properties is commensurate with their enormous variety of structural types.1 POMs can be derivatized with organic groups, and recent years have seen great advances in the synthetic chemistry of such POM “hybrids” both in terms of initial derivatization of the POM, and a range of organic transformations that can be used to postfunctionalize the resulting hybrid.2 These have allowed the construction of POM-organic architectures of increasing complexity, including polymers3 and POMs linked to catalytic centers4 or light-harvesting chromophores.5,6 Such research is motivated by the promise of emergent or synergic properties resulting from the combination of the organic and inorganic components
Our results show that imido-Lindqvist clusters are an unusually efficient acceptor for use with short π-bridges in second-order nonlinear optical (NLO) materials
With organic resonance electron donors, the resulting chromophores offer better transparency/ nonlinearity tradeoffs than similar purely organic systems and exceed empirical performance limits that apply to the vast majority of dipolar organic CT systems
Summary
Polyoxometalates (POMs) are a class of anionic, molecular metal oxide clusters, whose range of properties is commensurate with their enormous variety of structural types. POMs can be derivatized with organic groups, and recent years have seen great advances in the synthetic chemistry of such POM “hybrids” both in terms of initial derivatization of the POM, and a range of organic transformations that can be used to postfunctionalize the resulting hybrid. These have allowed the construction of POM-organic architectures of increasing complexity, including polymers and POMs linked to catalytic centers or light-harvesting chromophores. Such research is motivated by the promise of emergent or synergic properties resulting from the combination of the organic and inorganic components. We recently found that a small family of arylimido polyoxometalate derivatives have experimentally determined (by hyper-Rayleigh Scattering, HRS) static, resonance corrected β0 values of up to 133 × 10−30 esu.14 This compares to 25 × 10−30 esu for technologically valuable DAS+ under nonresonant conditions, and in some cases the POM-based chromophores break through empirical NLO performance limits for planar organics, as defined by electron-number adjusted β versus the absorption wavelength λmax. We expand the series of organoimido-POM chromophores, establishing structure−activity relationships, and use Resonance Raman and Stark spectroscopies, and density functional theory (DFT) calculations to elucidate the nature of the charge-transfer transitions responsible for their NLO properties These show that the derivatives lack dipolar character (and are NLO inactive) in the absence of resonance electron donors. Errors for Δμab, Hab, and cb are estimated as ±30%, ±30%, and ±50%, respectively
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